Nowadays, more and more investments in the advancement of electric motors are made in order to reach new levels of efficiency and achieve high energy savings even under demanding conditions. Electric motors play a key role in our
everyday businesses and lives. They move and run basically everything we need for business or pleasure. All these motors run on electricity – 28 to 30 percent of all electrical energy is converted to mechanical energy in electric motors.
In order to provide torque and speed, they need the corresponding amount of electric energy. A motor’s speed should match exactly what is required by the process in order to avoid dissipation of energy. By using more efficient motors it would be possible to realize huge savings in energy and carbon dioxide emissions.
Many types of electric motors are currently available for use and have to be considered in order to achieve best possible results. All different motor types have both advantages and disadvantages and it is important to evaluate them against the requirements of the specific application. It makes sense to have a closer look into the different motor technologies as choosing the right motor at the beginning can save a lot of energy and cost in the long run.
Induction motors are commonplace in industry due to their power and efficiency. The absence of commutator or brushes also make them reliable and relatively maintenance-free and there are constant efforts to make them increasingly efficient. However, these motors do have some drawbacks. The asynchronous speed results in conductor losses in the rotor that negatively affect efficiency, generate more heat and result in warmer bearings with a reduced lifetime.
The construction of PM motors is based on the standard induction motor design. The rotor magnetization is enabled by the use of permanent magnets which are mounted on the surface of the rotor or actually embedded within. The PM motor is synchronous, meaning that the rotor rotates in synchronism with the magnetic field. The motors can simplify drive systems by effectively eliminating the need of speed reduction devices and providing more precise speed control. They are designed exclusively for frequency converter supply, where they provide high speed accuracy even without speed sensors because they are synchronous motors without rotor slip. In addition, PM motors do not generate as much heat as IM, resulting in lower rotor/bearing temperature and thus in longer insulation and bearing lifetimes. PM motors provide more torque for the same size of package or the same amount of torque in a smaller package.
However, the use of rare-earth elements (REEs) is usually quite expensive and the costs can vary greatly. In addition, their strong magnetic rotor field can make servicing – a key feature of a mainstream industrial motor – more difficult. Another disadvantage is the generation of potentially dangerous voltages to the motor terminals due to the free movement of the motor shaft.
SynRM in combination with the today’s sophisticated VSD control electronics make it possible to fully exploit these super-efficient electrical machines. In SynRMs, the rotor is designed to produce the smallest possible magnetic reluctance (the resistance to the flow of a magnetic field) in one direction and the highest in the perpendicular direction. The rotor turns at the same frequency as the stator field (as in the permanent magnet motor).
SynRM perform better than conventional induction motors. They can be designed for highly efficient performance or to provide a higher power density for a smaller footprint than an equivalent IM. They need less maintenance, have a reduced inertia and are extremely reliable. Without magnets and without a cage, the rotor construction is simpler than either IMs or PM motors. The lower operating temperature of a SynRM has multiple benefits – including longer insulation life and extended bearing greasing intervals across the lifetime of the product, so avoiding motor outages.
A unique feature of this motor is that it uses ferrite (iron oxide, Fe2O) magnets, which are generally more cost-effective and more easily available than rare-earth permanent magnets. Their use results in a more economical and ecologically sustainable product. Ferrites have been used before in low power motor applications, but in industry a ferrite-based motor alone could not compete against an IM.
These motors are typically tightly integrated drive and motor combinations. The motor is often a ferrite synchronous motor, but sometimes other magnetic materials are used. It has similar characteristics to a PM motor as the rotor is running at the same speed as the magnetic field. The biggest differences are due to the tight integration of the drive electronics to the motor. The modulation of the drive and the output current of the inverter are optimized to the motor resulting in a compact package.
This means that the whole unit has to be replaced if anything fails, whether it is the bearings, capacitor, motor insulation or IGBT. Package designs are manufacturer specific, so sourcing the spare unit can take time. Additionally, the inverter is typically designed to be low cost, which means poorer performance with supply voltage sags and dips, higher harmonic content on the supply network.
| Title | Induction motor | Synchronous reluctance motor | Ferrite assisted synchronous reluctance motor | Permanent magnet motor | Electronically commutated motors |
| Typical power range | Wide power range | IE5 SynRM 5.5 – 315 kW (ABB) | Focus 0.55 – 18.5 kW (ABB) | Wide power range (depending on manufacturer) |
Typically up to 15 kW |
| Typical efficiency range | IE3 and IE4 | Up to IE5 | Up to IE5+ | Up to IE4 | Typically up to IE4 |
| Speed range above FWP | Up to 2 x nominal speed | 1.4 x nominal speed or more | Up to 1.5 – 2 x nominal speed | Typically up to 1.2 x nominal speed | 1.0 x nominal spee |
| DOL/VSD | DOL and VSD | VSD, special control SW needed | VSD, special control SW needed | VSD, special control SW needed | Built-in VSD for speed control is required |
| IEC frame comparison to IE2 induction | IE3 and IE4 typically bigger | Same or smaller | Smaller | Same or smaller | Typically shorter with wider diameter, tailored to the application mechanics |
| Applications | All industrial applications such as pumps, fans, compressors, conveyors, extruders, winches, cranes |
Most industrial applications including pumps,fans,compressors,conveyors,extruders | Ideal for applications with highest efficiency demands |
Most industrial applications where high efficiency is important |
Pumps, fans |
| Advantage | – Well-known, robust and proven technology -Simple and easy to maintain |
-High efficiency and reliability -Higher power density -Cool motor -Lower bearing temperature and longer bearing lifetime – No rotor-cage – Magnet-free, can be controlled without encoders – Cost-efficient solution |
-Highest efficiency class – High power factor and low current demand – Ferrites are more cost-efficient than rare-earth permanent magnets |
-Significant energy saving potential – Permanent magnets reduce rotor losses and increase motor efficiency – Compact motor – Low noise levels – Low bearing temperature |
-One package with everything integrated – Quick to install with only power and reference or Modbus connection required |
| Disadvantage | – Difficult to reach highest efficiency levels – Higher bearing temperature compared to others |
– Difficult to reach highest efficiency levels – Higher bearing temperature compared to others |
– Generates voltage on the terminals without locked shaft – To perform system maintenance, the installer needs to know that the shaft cannot rotate eg, due to airflow in duct |
-Expensive rare-earth PM materials -Generates dangerous voltage on the terminals without locked shaft – To perform system maintenance, the installer needs to know that the shaft cannot rotate eg, due to airflow in duct |
– No application specific functionality (e.g. flying start) – Limited undervoltage performance to voltage dips – High harmonics (typically comparable to no-choke VSD) – Not stocked universally, longer delivery times – Lack of BACnet support makes integration to building automation more difficult (only Modbus RTU support) – No smoke exhaust certification |
| Maintenance | – Easy – No magnetic forces – Test run can be done direct-on-line – Universally available from anywhere |
– Easy – No magnetic forces – Test run requires a drive |
– Easy – Low magnetic forces – Embedded magnets, no magnet damage during rotor removal – Test run requires a drive |
– Difficult – Strong magnetic forces – Removing rotor from the stator is difficult and requires special tools – Potential magnet damage in case of surface mounted magnets – Test run requires a drive |
– Replace everything at once if any sub component such as bearings, semiconductors,
capacitors, motor insulation, etc. fails – Not stocked widely, longer lead times |
